Oxygen isotopic compositions of Allende Type C CAIs : Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

摘要: Abstract In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that pattern relative enrichments depletions 16 O within each individual CAI are similar to patterns observed Types A B same meteorite. Spinel is consistently most O-rich (Δ 17 O = −25‰ −15‰), followed by Al,Ti-dioside O = −20‰ −5‰) anorthite O = −15‰ 0‰). Melilite O-depleted mineral O = −5‰ −3‰). We conclude original melting event formed occurred a O ⩽ −20‰) nebular gas they subsequently experienced exchange O-poor reservoir. At least three these ( ABC , TS26F1 93 ) remelting at time place where chondrules were forming, trapping partially assimilating chondrule fragments. The observation pyroxene feldspar, even though feldspar preceded it igneous crystallization sequence, disproves class models which partial solid slow gas. For typical (not associated with materials) as well for CAIs, produced internal heterogeneity must have largely state. phases grossular, monticellite forsterite commonly compositions melilite replace, but one case (CAI 160 grossular O-enriched O = −10‰ −6‰) −3‰), meaning exchanged its subsequent alteration. This likely on CV parent asteroid, possibly during fluid-assisted thermal metamorphism.